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Screened or unscreened? We explore the options for Cat 6a installations

It's a popular debate in our current market deciding on whether Class Ea or Category 6a cable installations should be screened to support 10Gig Ethernet transmission. And it's a good question. We take a look at the advantages and disadvantages of both screened and unscreened cabling in this article and hope that it offers a better understanding for making an installation decision.

A global standard since the 1980s, screened and shielded twisted-pair copper cabling varieties are still a mainstay in some markets. While many markets have migrated largely to unshielded twisted-pair (U/UTP) cables, the ratification of the 10GBase-I standard for 10-Gbit/sec Ethernet over copper has re-established the commercial viability of screened and shielded systems, and has fueled greater adoption of these systems in markets that previously were UTP-centric.

It has been shown that the introduction of 10GBase-T in fact has a considerable impact on the selection of cabling. The increased sensitivity of 10GBase-T transmissions compared to 1000Base-T was clearly evident with unshielded cabling in terms of immunity against external interference. In 10GBASE-T applications, the noise source that most limits the ability to transmit 10Gb Ethernet over copper cabling is alien crosstalk. Because the 10GBASE-T receiver cannot compensate for the noise from adjacent channels, this effect must be cancelled out wherever possible by the cabling system to ensure reliable data transmission. This noise is measured as Power Sum Alien Near-End Crosstalk (PSANEXT) and as Power Sum Alien Attenuation to Crosstalk Ratio at the Far-End (PSAACRF). Both ISO/IEC 11801 Ed 2.2 Class EA and TIA-EIA-568-C.2 Category 6A require that crosstalk be measured in a 6-around-1 cabling configuration that takes into account the worst-case effect on a centre cable with six cables tightly bundled around it. A Category 6 U/UTP system will not meet the alien crosstalk limits required for 100 metres of 10GBASE-T transmission.

Following on from the introduction, there are a number of considerations to be made when deciding on either a screened or unscreened solution for your cabling solution, including the budget available; the performance of the cabling system; the electrical environment in to which the cabling will be installed (the level and type of Electromagnetic Interference (EMI); the type of cable containment; the proximity of data cables to power cables and the availability of adequate points of earthing within the building to be cabled.

In properly installed and bonded screened cabling, the foil screen within the cable prevents signals from coupling which reduces alien crosstalk well below the required limits. All the tests mentioned earlier indicate that screened cabling systems provide significant margin over the IEEE 802.3an-2006 specification for 10GBase-T PSANEXT and PSACCRF, thereby removing the need for time-consuming and complicated field-testing of alien crosstalk completely. Therefore ISO 11801 clearly states that alien crosstalk testing is NOT required for screened systems, making this a cost effective and also time effective choice.

Performing a 100% alien crosstalk test in a cabling plant is impractical and virtually impossible in large cabling plants. Using the specified 6-around-1 method, the formula to determine the number of tests that would need to be run for 100% coverage is (n2+n)/2 where n is the number of links in the installation. 

For example, in an installation with 100 links, a total of 5,050 tests would need to be run to test every possible combination. In a 500-link installation the total number of tests climbs to 125,250 tests when testing every possible combination. That's a lot of time and a lot of money! 

​An independent test of five different Class Ea Cabling Systems (including both screened and unscreened solutions) from five different market-leading suppliers found that in all cases, the screened solutions provided greater headroom than the unscreened systems. The second factor was the unscreened systems tested demonstrated significantly weaker ANEXT performance and coupling attenuation in comparison to the screened systems. 

Other tests included immunity against fast transient electrical disturbances, such as Powering of Fluorescent Lamps and immunity against radiated electromagnetic fields, such as those produced by GSM based mobile phones. Once again the unscreened systems performed badly in comparison to the screened systems.

The key weakness of a U/UTP system comes about when you have a large quantity of adjacent ports loaded into patch panels, a fact that is highlighted within the measurement of ANEXT within ISO/IEC 11801 ed2.2 as by definition it does not meet the criteria of the infrastructure design element.

​As many of us now know, the use of PoE has the side effect of heating up bundles of cables. As the demand for higher levels of power increases, so does the level of heating; which in turn is one of the major contributors for the increase in attenuation. What many may not realise is the true extent of this and the fact that it differs between unscreened and screened cabling.

The performance criteria for the 100m Channel as outlined in EN 50173-2 is based upon it operating at an ambient temperature of 20˚C and for every degree over this level this distance should be reduced. The following formula provided in the above standard gives the rate of reduction for unscreened cables. In short for temperature increases up to 20˚C above the ambient the Channel should be reduced by 4% and for temperatures over 20˚C above the ambient, there is an additional 6% that has to be added.

Unscreened

Lt>20˚C=L/(1 + (T-20) x 0,004)

Lt>40˚C=L/(1 + (T-20) x 0,004 + (T-40) x 0.006)

Screened

Lt>20˚C=L/(1 + (T-20) *0,002)

This could potentially have a dramatic effect to the performance of installed cabling as recent research shows that the level of heating can be significant in some cases 30-40˚C above the ambient. Again screened cabling performs much better, firstly research has proved it does not heat up as much as an unscreened cable and when it does the de-rating formula is much simpler as it is based upon 2%.

With all of the above in mind, we take a look at the differences and myths between Screened and Unscreened:

​No screen, simpler and quicker to terminate. Yes and No - more care is needed in the preparation to ensure twist ratios are maintained etc. Most U/UTP solutions are very tightly twisted pairs and a large plastic separator. ​Most manufacturers offer either a termination aid or have toolless products which lead to the overall time taken being quicker than U/UTP. Certainly the cable pulling time will not change.
​Cable pulling time for an unscreened solution can vary from slightly to a lot worse depending on the actual construction of the cable. ​Most screened cables have a relaxed twist on each of the pairs meaning that the cable itself is much less stiff and easier to handle and install.
​Does not require Bonding. This is a Myth. All metal panels within a cabinet whether screened or unscreened need to be bonded within the cabinet in accordance with BS/EN50310.​A small amount of additional time is required to ensure all the outlets within each panel have a clean contact with the frame.
​UTP cables are smaller. Again a myth. Some U/UTP cable have an elliptical design and overall OD which is on average anywhere between 7.3 - 9.3mm, depending upon the manufacturer, however they are all bigger requiring more containment, larger bends and larger back boxes.​Average size of an Excel F/FTP solution is 6.9mm U/FTP is 6.7mm. The U/FTP cable is also available in a 305m box, thereby reducing set up time for cable pulling by as much as 75%. For the same physical space, it is possible to get as many as 15% more cables in the same space based upon the smallest U/UTP available from a leading manufacturer.
​Field Testing - Although not common alien cross talk testing can be requested requiring a 6 around 1 test method. A number of field testers make assumptions for this and rely on the manufacturer to back them up. If the full 6 around 1 test is called for, the additional time for testing is a minimum 10-15 mins per link. This is separate and on top of the Permanent Link Testing.​ANEXT testing is not required, typical test time for a permanent link is approx 14-22 seconds, although there are next generation testers on the market that can test a Permanent Link less than 10 seconds.
​Separation distances between Power and data are greatly increased with an unscreened cable e.g. for 10 circuits of 20A there has to be a physical separation between the power and the data cables of 80mm.​The separation distances between the same number of power circuits is at least halved with foil screening requiring a distance of only 40mm and a S/FTP construction requiring even less.
​Increased attenuation caused by temperature. Unscreened cable has a higher and more complex de-rating factor.​Increased attenuation caused by temperature. Screened cable has a lower and simpler de-rating factor.

From all of the information given, it is evident that screened is best when considering a Category 6a cable installation. However, unscreened can be a viable option for those that choose to take that route, and for those that understand the implications as outlined above. One thing that is becoming clear, is that the number of companies choosing a screened solution is dramatically increasing across the globe, even in markets that have been firmly unscreened historically. This most likely is through better understanding and education of the two options and therefore making a more informed decision between the two.


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